1. Field of the Invention
The present invention relates to a coupling device for transmitting torque.
2. Description of Related Art
JP-A-8-319945 discloses a compressor with coupling device. The coupling device transmits torque. The coupling device is formed as a pulley for receiving rotational torque from a belt. The coupling device is also capable of functioning as a torque limiter to disconnect torque transmission when the torque exceeds a predetermined threshold torque. The coupling device is formed with a weak portion that breaks to disconnect torque transmission when the torque exceeds the threshold. The weak portion is provided on a disk portion of the pulley by forming an annular groove and through holes on the groove in regular intervals. The through hole weakens and adjusts the strength of the disk. This configuration is advantageous for providing a coupling device with simple structure and at less cost. However, contrary, it is difficult to design and manufacture products having precision break torque.
For instance, a breakable portion expected to be broken has to be designed to have a certain strength that is capable to breaking at a certain break torque, threshold torque. However, it is difficult to determine the strength of the breakable portion. For example, the fatigue failure may breaks the breakable portion even if it is applied with a lower toque than the expected break torque.
Therefore, the maximum torque applied to the breakable portion, that is a permissible torque T2, should be set below a torque indicated by T1/S, where T1 is the break torque and S is a safety margin. In this case, the coupling device will be useless since the required transmissible torque T3 exceeds the permissible torque T2 when a ratio T1/T3 is lower than the safety ratio S. The required transmissible torque T3 may be referred to as a torque to be transmitted via the coupling device under an actual application. In general, it is necessary to set a greater value in the safety margin S as it becomes difficult to carry out a theoretical stress analysis of the object since condition of stress applied to the abject requires complicated analysis. In contrast, if the stress applied to the breakable portion can be estimated precisely, it is possible to set a smaller value in the safety margin S, and to increase the permissible torque T2.
The structure disclosed in JP-A-8-319945 is expected to be broken primarily by a shearing force, that is the tangential stress. Since the shearing force tends to intense to a surface, it is difficult to estimate a distribution of the stress precisely. Therefore, a trial and error method is required to determine appropriate size and material of the breakable portion.